Flow Misalignment and Tidal Stream Turbines

Abstract

Extensive Research and Development (R&D) within the tidal energy industry is pushing this sector towards commercial viability, with full scale prototypes starting to meet the challenges of the marine environment. To date, the focus has been on Horizontal Axis Tidal Turbines (HATTs); comprising of a turbine supported by a tubular stanchion operating on a bi-directional, or yawing principle. This is in order to accommodate the directional spread of the tidal flow as it varies over the tidal cycle. This paper combines velocity data measured in Ramsey Sound (Pembrokeshire, Wales, UK), a macrotidal strait with Computational Fluid Dynamics (CFD) to assess the impact of non-rectilinear flows on turbine rotor performance. In order to do this both the geometry of the turbine and the surrounding free stream velocity needs to be studied. It was deemed from literature and the site data that the majority of the velocities tend to fall within a ±20° misalignment to the principle flow direction for velocities greater than the economic viable threshold of 2 ms-1. From the CFD it was found that an axial flow misalignment of +10° results in a 7.5% reduction in peak CP, 3% in peak C$θ$ and 9% in peak CT, compared to the aligned turbine. The +20° misaligned turbine experienced a drop of 29% in peak CP, 11% in peak C$θ$ and 24.5% in peak CT, compared to the aligned turbine. The CM about the head of the driveshaft for the 10° and 20° misaligned turbines were found to be five times and nine times greater, respectively, than the aligned turbine. The work has shown the detrimental impact of operating a turbine in a high velocity misaligned flow regime. The paper shows that the tolerance to axial flow misalignment between the free stream velocity and axis of rotation of a HATT requires defining, in order to avoid the detrimental effects it has on performance and loading.